How do you get a ball belly?

When a person, typically a male, gains so much weight his stomach becomes circular. And his stomach literally looks like a ball.

The cartoon is a bit of an exaggeration, but it's very close to what a ball belly looks like.

http://1.bp.blogspot.com/_aKuV0NZVE9Q/R_G2o3pm8xI/AAAAAAAAAA0/wNwdQZxi65s/s320/bigbelly2.jpg